1. Field of the Invention
The present invention relates generally to a method for forming the phase shift mask of photolithography process, and more particularly to a method for fabricating alternating phase shift mask of photolithography process with ion-implanted layer.
2. Description of the Prior Art
As semiconductor devices, such as the Metal-Oxide-Semiconductor device, become highly integrated the area occupied by the device shrinks, as well as the design rule. With advances in the semiconductor technology, the dimensions of the integrated circuit (IC) devices have shrunk to the deep sub-micron range. As the semiconductor device continuously shrinks in the deep sub-micron region, some problems described below are incurred due to the scaling down process. In order to build very small electrically active devices on the wafer, the impact of these operations has to be confined to small, well-defined regions.
Lithography in the context of VLSI manufacturing of CMOS devices is the process of patterning openings in photosensitive polymers (sometimes referred to as photoresists or resists) which define small areas in which the silicon base (or other) material is modified by a specific operation in a sequence of processing steps. The manufacturing of CMOS chips involves the repeated patterning of photoresist, followed by an etch, implant, deposition, or other operation, and ending in the removal of the expended photoresist to make way for a new resist to be applied for another iteration of this process sequence. The basic lithography system consists of a light source, a stencil or photomask containing the pattern to be transferred to the wafer, a collection of lenses, and a means for aligning existing patterns on the wafer with patterns on the mask. A lithography stepper is limited by parameters described in Rayleigh's equation:R=k1*λ/NAWherein, λ is the wavelength of the light source used in the projection system and NA is the numerical aperture of the projection optics used. k1 is a factor describing how well a combined lithography system can utilize the theoretical resolution limit in practice and can range from 0.8 down to 0.5 for standard exposure systems. The highest resolution in optical lithography is currently achieved with deep ultra violet (DUV) steppers operating at 248 nm wavelength. Steppers operating at a wavelength of 356 nm are also in widespread use.
Conventional photomask consist of chromium patterns on a quartz plate, allowing light to pass wherever the chromium is removed from the mask. Light of a specific wavelength is projected through a mask onto the photoresist coated wafer, exposing the resist wherever hole patterns are placed on the mask. Exposing the resist to light of appropriate wavelength causes modifications in the molecular structure of the resist polymers which allows a developer chemical to dissolve and remove the resist in the exposed areas. (Conversely, negative resist systems allow only unexposed resist to be developed away.) The photomask, when illuminated, can be pictured as an array of individual, infinitely small light sources which can be either turned on (points covered by clear areas) or turned off (points covered by chrome). These conventional photomasks are commonly referred to as chrome on glass (COG) binary masks. The perfectly square step function exists only in the theoretical limit of the exact mask plane. At any distance away from the mask, such as in the wafer plane, diffraction effects will cause images to exhibit a finite image slope. At small dimensions, that is, when the size and spacing of the images to be printed are small relative to λ/NA (NA being the numerical aperture of the exposure system), electric field vectors of nearby images will interact and add constructively. The resulting light intensity curve between features is not completely dark, but exhibits significant amounts of light intensity created by the interaction of adjacent features. The resolution of an exposure system is limited by the contrast of the projected light image, which is the intensity difference between adjacent light and dark features. An increase in the light intensity in nominally dark regions will eventually cause adjacent features to print as one combined structure rather than discrete images.
The quality with which small images can be replicated in lithography depends largely on the available process latitude, that is, the amount of allowable dose and focus variation that still results in correct image size. As design feature are rapid shrinking, all of the lithography resolution enhancement techniques (RET), in principle, the use of Alternating Phase Mask (Strong Phase Shifted Mask) is the most effective method for it provides a nearly doubled resolution enhancement of patterning. Phase shifted mask (PSM) lithography improves the lithographic process latitude or allows operation of a lower k.sub. 1 value (see equation 1) by introducing a third parameter on the mask. The electric field vector, like any vector quantity, has a magnitude and direction, so in addition to turning the electric field amplitude on and off, the phase of the vector can changed. This phase variation is achieved in PSM's by modifying the length that a light beam travels through the mask material. By recessing the mask by the appropriate depth, light traversing the thinner portion of the mask and light traversing the thicker portion of the mask will be π out of phase; that is, their electric field vectors will be of equal magnitude but point in exactly opposite directions so that any interaction between these light beams results in perfect cancellation.
The conventional mask is made of quartz with an image in chrome. This is referred to as a “chrome on glass” or binary mask. The minimum dimensions of circuits formed by photolithographic processes generally decrease as improvements in semiconductor manufacturing processes occur. Improving photolithography technology provides improved resolution, resulting in a potential reduction of the minimum dimensions of and spaces between electromagnetic radiation application regions where electromagnetic radiation is applied through the mask. However, conventional phase mask is difficult to be fabricated for forming the uniform intensity of exposure light. Except the difficulty of mask manufacturing, the most problem is the aerial intensity imbalance between the shift and unshifted region. It is because the exposure light was affected by scattering effect of mask topography, and then the diffraction light will results in different transmission intensity on wafer.
In accordance with the above description, a new and improved method for forming the phase mask is therefore necessary, so as to raise the yield and quality of the follow-up process.